Lateral fluxes of biogenic particles through the Mien-Hua canyon in the southern East China Sea slope

Abstract

Sediment traps were deployed on the southern East China Sea (ECS) continental slope to measure particle and biogenic fluxes for deriving particulate carbon exports from the ECS continental shelf through the Mien-Hua Canyon (MHC). Particle fluxes are much higher in the MHC than in other non-canyon slopes, ranging from 4.98 g m −2 d −1 on the lower slope to 71.5 g m −2 d −1 in the MHC, indicating that the MHC is a major conduit of particle transport. More than 80% of collected particles are lithogenic, regardless of trap sites. Biogenic contents are diluted by lithogenic materials and therefore decrease with the increase in mass fluxes. However, biogenic fluxes remain significant as they are proportional to mass fluxes, ranging from 21 to 368 mg C m −2 d −1 for particulate organic carbon (POC), 12–750 mg C m −2 d −1 for particulate inorganic carbon (PIC), and 21–755 mg m −2 d −1 for opal. Although particle and biogenic fluxes decrease with distance away from the shelf break, these fluxes increase significantly with water depth, indicating the important contribution of lateral transport to measured fluxes. Most POC collected from the slope is likely derived either from the shelf or from the upper-middle slope. Short-term variabilities of collected biogenic fluxes are likely linked to episodic resuspension and lateral movement from the shelf to the slope. Such cross-shelf and off-slope-directed transport of biogenic particles appears to function via the bottom and intermediate nepheloid layers over the shelf and slope. Total shelf fluxes through the MHC are estimated to be 2.43×10 11 g C yr −1 for POC and 4.20×10 11 g C yr −1 for PIC; the POC flux accounts for 0.17% of total primary production on the shelf. If these exports are extrapolated to calculate the amount of POC and PIC export through the whole ECS slope, the exported POC and PIC may be estimated to be less than 9.35×10 12 and 16.2×10 12 g C yr −1, respectively. The shelf export of POC is equivalent to <6.4% of carbon fixed biologically on the shelf. The export of particulate nitrogen is estimated to be less than 1.18×10 12 g N yr −1, that is equivalent to <4.6% of total nitrogen required to sustain primary production on the ECS shelf. More than 70% of the nitrogen used for new production (4.11×10 12 g N yr −1) may be buried in shelf sediments.